Hello. I'm working on formatting my first ebook. I'm pasting it up in InDesign and using its EPUB export function.

My book is in English but contains occasional text in Japanese; the font I'm using in InDesign is Hiragino Mincho ProN W3.

When I export the EPUB without embedded fonts, the Japanese glyphs don't render in ADE, but they look fine in iBooks and Calibre. With embedded fonts, the glyphs render fine in ADE--although if I edit the EPUB in Sigil and then reopen it in ADE, the glyphs are broken again.

I'd prefer not to embed fonts unless there's a good reason to. Are there any common platforms that will fail to render my Japanese text if I don't embed fonts? Do I need to worry about ADE? Does anyone even use it anymore? What's the best practice for this situation?

The majority of the e-ink readers use the mobile version of ADE, so yes, a lot of readers use ADE. You are probably better of with an embedded version, which is totally possible, also with Sigil. The problem is, afak, that Indesign does some akward things.
Also, keep in mind that the OS/2 version of the embedded font must be 3 or lower. Not 4 or automatic.

Fine. Usage of some Japanese characters in an otherwise Latin-based text seems a good reason to me (with the current generation of ebook readers, at least). I assume you mean hiragana/katakana/kanji characters, and not just Latin letters with diacritics used for transliteration. If it's the latter, then it's not so clear... but still it's a valid reason.

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Are there any common platforms that will fail to render my Japanese text if I don't embed fonts?

Yes. ADE, as you found out, and most dedicated hardware ebook readers, which are based on the same rendering engine as ADE.

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Do I need to worry about ADE?

About ADE itself, maybe not so much. But it's probably the best "emulator" for ebook readers.

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Does anyone even use it anymore?

Well... almost everyone who buys ePub books has to use ADE for downloading them. But see above, anyway.

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What's the best practice for this situation?

You cannot expect the default fonts in every reader to include all possible Unicode characters. So as soon as you are including some exotic ones (and Japanese is exotic in most American and European countries) it's a good idea to embed a font. But code the CSS such that it's very simple to disable the font, so an interested user (or yourself in the future) can easily do that. With this I mean, for instance, using the embedded font in a single class, and using multiple classes in HTML elements if needed, rather than using the embedded font in multiple classes.

Here's the problem. Editing an EPUB in Sigil or otherwise seems to wreck Adobe's obfuscated embedded fonts, at least as far as ADE is concerned. So I downloaded an open source Japanese font (Hanazono) and embedded it manually by just dragging the (18.5MB) font file into Sigil.

This worked fine--I can edit the stylesheet all I want and it still loads perfectly in ADE and everything else I test it on. However, now the EPUB file is enormous. So I want to subset the font. I tried using the glyphIgo script which I found on this forum, but it didn't work: it only saw unicode in the latin range in my EPUB, despite the presence of Japanese characters.

Has anyone gotten that script to work? Any ideas for subsetting the Japanese font without having InDesign mangle it? Thanks again.